GLUCOSE DECREASES INTRACELLULAR FREE CALCIUM AND PH IN CULTURED RAT AORTIC VASCULAR SMOOTH-MUSCLE CELLS - ROLE OF CA2+-ATPASE

Early diabetes mellitus leading to hyperglycemia in both animals and man is associated with impaired blood flow autoregulation. The mechanisms underlying such alterations, however, are unclear, although a role for VSM cells and alterations in [Ca2+](i) homeostasis are likely candidates. The present study investigates the effect of elevated extracellular glucose concentrations on [Ca2+](i) and intracellular pH (pH(i)) in cultured rat aortic VSM cells. The VSM cells were coloaded with Fura-2 and SNARF-1 for simultaneous [Ca2+](i) and pH(i) measurements, respectively. VSM cells were treated with glucose concentrations from 0 to 23 mM The results indicate that glucose induces a decrease in both [Ca2+](i) and pH(i) in a concentration-dependent manner. The effects of glucose on [Ca2+](i) and pH(i) were fully reversible. To investigate whether the [Ca2+](i) decreases were due to activation of Ca2+-ATPase by glucose, we used Ca2+-ATPase inhibitors: thapsigargin (TG), cyclopiazonic acid (CPA) and 2,5-di-(tert-butyl)-1,4-benzo-hydroquinone (BHQ). Pretreatment with either TG, CPA, or BHQ abolished the glucose-induced [Ca2+](i) and pH(i) decrease. The effect of drugs on [Ca2+](i) and pH(i) following glucose was drug-dependent. These data indicate that glucose and [Ca2+](i) are linked, and may be explained by direct coupling of glycolysis to the activity of Ca2+-ATPase. We concluded that: (1) VSM cells respond to glucose treatment with rapid and sustained decreases in both pH(l) and [Ca2+](i); (2) the magnitudes of the effects of glucose on pH(l) and [Ca2+](i) were concentration dependent and fully reversible, and (3) the glucose-induced [Ca2+](i) decrease was abolished by Ca2+ ATPase inhibitors. The alterations in pH(i) and [Ca2+](i) induced by glucose may be relevant for the decrease in vascular tone observed in early diabetes mellitus.